1. Field of the Invention
The present invention generally relates to apparatus employing charged particle beams and, more particularly, to ion beam apparatus employing magnets to manipulate the ion beam.
2. Description of the Prior Art
The art of semiconductor electronic device manufacture has become highly sophisticated in recent years to provide a wide range of electrical properties of the devices, often at very high integration density. The capability to determine the electrical properties with high reliability, consistency and manufacturing yield is often limited, as a practical matter, by the tools used for processing the semiconductor material, usually in the form of a wafer. Such tools are often complex and of high precision. Therefore such tools are generally expensive to build and maintain. The principal expense of modern semiconductor devices is thus a portion of the cost of the tools used to produce them and, therefore, varies inversely with tool throughput.
As is generally known, pure semiconductor materials are poor conductors of electricity but, as such, the electrical properties of semiconductor materials can be altered radically by impurities and/or electrical fields established therein; the latter being generally used for control of the device while the former is generally used for establishing device specifications. Impurities can be introduced into semiconductor materials either during growth or deposition or by implantation. Implantation is often preferred for high precision of placement of the impurities and process simplicity. That is, implantation of particles in the form of ions can accurately place impurities at a desired depth within an existing structure in accordance with the energy (or, more accurately, the distribution of energies) imparted to the particles to be implanted and the nature of the material in which the particles are to be implanted. By the same token, impurities may be implanted into an existing structure in a single process whereas at least two growth or deposition processes would be required to form a buried layer having impurities therein.
It should be appreciated that the distribution of energies of the particles, often referred to simply as beam energy, is dictated by the device design (e.g. where the impurities are to be placed) as is the concentration of impurities to be achieved. The desired concentration of impurities is determined as a function of charged particle flux at the surface of the semiconductor material or target and the duration of the implantation process. It follows that the energy of the beam cannot be altered to increase particle flux and thereby reduce the duration of the implantation process. Accordingly, the desired concentrations of impurities may require substantial time to achieve; thereby reducing tool throughput and increasing expense.
Unfortunately, several physical mechanisms of ion beams tend to substantially reduce ion flux. Specifically, it is common practice to use a magnetic field to control or manipulate the ion beam. One particular such manipulation is referred to as mass analysis. In the mass analysis process, ions will have the same charge and their motion along the beam path represents a current. Therefore, when such charged particles pass through a magnetic dipole, a force is exerted on each ion perpendicular to both. the direction of the beam and the direction of the magnetic field. Due to this force, the trajectory of each ion is altered to a degree inversely proportional to the square root of its mass. This effect allows removal of ions from the beam which are not of the desired material and the remainder of the beam will be limited to ions of a particular mass. This type of structure is routinely included in ion beam tools for that reason and the reduction of ion flux in the beam by removal of ions of undesired materials is not of concern.
However, in the magnetic dipole gap of the mass analysis magnet or any other magnet in the tool, the ion beam tends to diverge significantly in a manner similar to effects of Coulomb interactions between ions (sometimes referred to as "space charge blow up") even when the ion beam energy is sufficiently great to create a plasma within the magnet. (Presence of a plasma including free electrons tends to reduce the repulsion forces between ions in the beam, sometimes referred to as space charge neutralization.) Even though the mechanism of beam divergence may not be fully or accurately understood, beam divergence within the magnet is known to be significantly greater than in a comparable length of unmagnetized beam line (provided there are no electrodes along that length of beam line that would destroy the beam plasma). It is generally believed, however, that the increase in beam divergence is due to an increase in electron temperature within the beam plasma in the magnet relative to plasma electron temperature outside the magnet.
The divergence of the beam within the magnet is also principally in the direction of the magnetic field (e.g. across the gap between the pole pieces) and, at the same time, the transverse size of the beam is limited by the size of the pole gap thus reducing flux by the truncation of the edges of the beam as ions impinge upon the pole pieces. The beam divergence increases with increased ion beam current and decreased beam energy. At low beam energies, the ion beam is less effective to produce ionization which would, in turn, produce a beam plasma that partially compensates for the space charge of the ion beam. Therefore, it can be seen that seeking to increase tool throughput by increasing beam current provides only marginal, if any, advantage since increased beam current increases beam divergence and loss of ion flux in the magnet and at the target which largely counteracts the increase of beam current. Further, the spreading effect is aggravated at low beam energies and particularly at high currents.
In a plasma outside of a dipole magnet, it is well-known to confine the plasma and reduce the electron temperature by confining the electrons magnetically with a multi-pole magnetic structure. In such a structure, the electrons are confined by a large mirror ratio at magnetic cusps. However, when a multi-pole field is combined with a dipole field as disclosed, for example, in U.S. Pat. No. 5,206,516, either the electrons are confined by a cusp at one side of the dipole field but the field is decreased at the other side of the dipole field or, if the electrons are confined by cusps at both sides of the dipole field, the region of the cusps will be followed, along the beam line, by a region of reduced magnetic field. Thus, very little net confinement is achieved in either case.
Accordingly, it is seen that there has been, prior to the present invention, no known technique for increasing ion beam tool throughput since neither increase of ion beam current nor magnetic confinement with a multi-pole structure provides a significant increase in ion beam flux at the target.